Metal working fluids are used in machining and working operations of metals, where they serve to reduce friction, improve cooling, reduce corrosion, reduce wear, or otherwise improve the machining process. Typical metal working processes in which metal working fluids are often used include cutting, drilling, tapping, grinding, milling, turning, punching, stamping, rolling and like operations.
Metal working fluids are available in synthetic, semi-synthetic and soluble oil forms. Synthetic systems comprise aqueous dispersions of components to act as lubricating agents, corrosion inhibitors, anti-foaming agents and the like. They typically comprise less than 5% w/w mineral oils or related oils. Semi-synthetic systems comprise emulsions of oils, e.g. mineral oils, in aqueous solutions of emulsifying agents. The oil is highly dispersed in the aqueous system through the action of the surfactants. Typically, semi-synthetic metal working fluids contain from 5 to 40% w/w mineral oil. Soluble oil metal working fluids typically contain more than 40% w/w oil and often contain little or no water. Soluble oil fluids are preferred to synthetic forms in many circumstances because of their better cutting performance.
Metal working fluids are usually supplied as a concentrate and then diluted in water before being used. Additional additives such as corrosion inhibitors will also routinely be added to the concentrate or the final product formulations. Block copolymers (BCP), and in particular reverse block co-polymers (RBCP), are well known for use in metal working fluids, where they act as lubricants and often as surfactants. They are used in fluids for the working of steel, aluminium and many other alloys and metals. Typically RBCP containing compositions are provided as a water-based/water-dilutable concentrate. Though RBCP-based compositions tend to perform adequately in machining of steel, they are known to perform poorly in machining of aluminium. There is a need in the industry for improved compositions for use in metal working fluids, which perform well when working aluminium, and other light weight metals. This is apparent from the recent increases in the industrial use of aluminium and other light weight metals and alloys, which have become as important as, if not more important than, the use of steel. There is an even greater need for metal working fluids which are effective in working both steel and aluminium, and preferably other metals and alloys.
There is also a desire in industry for compositions which allow the user to view the workpiece while the metal working fluid is being applied. This requires that the metal working fluid is relatively transparent or translucent. In synthetic or semi-synthetic metal working fluids it is extremely difficult to achieve optimum cutting performance in such a metal working fluid. This is due to the fact that typically the metal working fluid clarity is inversely proportional to the cutting efficiency, whereby coarse, opaque, large particle size fluids outperform small particle size clear fluids.
In the case of RBCPs, fluid clarity or translucency is, at least in part, dependent upon the cloud point temperature of a material. Cloud point temperature is the temperature at which cloudiness becomes apparent in a liquid as the temperature is raised; i.e., the temperature at which turbidity is first noted in a liquid. Usually this is the result of the separation of the solute from water in a reaction mixture. It has been standard practice to market copolymer-containing metal working fluid concentrates that separate into two or more phases at elevated storage temperatures, these temperatures are often around 50° C. This separation is the result of the low cloud point temperatures of RBCP. Typically RBCP containing compositions have cloud point temperatures of from around 25 to 35° C.
Polyalkylene glycols in general, and RBCPs in particular, tend to phase separate and become concentrated at the hot tool/workpiece interface during the metal working process; these localised concentrations of RBCP generally provide excellent lubrication in steel metal working operations, but less so in aluminium. It is known that RBCPs exhibit polar attractions between their polyalkylene glycols and many metal surfaces, and this may account for some aspects of their performance as metal working fluids. Clouding out at the tool/workpiece interface is less of a problem in terms of visibility than a generally opaque composition.
It would be highly desirable to have an RBCP-containing formulation that retains its lubricity and does not separate as readily as known systems. However, given the highly effective nature of RBCPs for metal working, this stability deficiency has been tolerated in industry.
It would also be highly desirable to have more effective metal working fluids. In particular, this is a longstanding issue in aluminium working, where current systems generally perform very poorly.
The use of esters, partial esters and complex ester compositions are also known to be used in synthetic metal working fluid systems. These ester ingredients can provide desirable properties, such as better bio-stability, meaning the formulation is resistant to microbial contamination thus helping to extend the shelf life of the system. Metal working fluids are today being applied to metal workpieces with increasing velocity to maximize heat rejection during the processing operations. At the same time, sump sizes are getting smaller to minimize space and fluid inventory. With both of these features, foaming is becoming increasingly problematic, as foamed fluid is known to cause overflow problems with the sump. So it would be highly desirable to minimize the propensity to form foam in use. Furthermore low foaming is desired to ensure maximum contact between the tool and workpiece surface.
Traditionally, the processing of aluminium workpieces using synthetic or semi-synthetic metal working fluids results in partial darkening or staining of the workpieces as a result of oxidation and residual impurities on the workpieces. This darkening or staining negatively impacts further processing as the aluminium parts need to be cleaned in steps following the metal working, which adds cost to the machining process. Avoiding this staining in the first place would minimize or eliminate the additional processing steps, thereby increasing throughput and minimizing overall costs of the machining operation.
An effective metal working fluid desirably combines a number of properties to be suitable for a specific application. These properties commonly include lubricity, effective cooling, bio-stability, low foaming to ensure maximum contact between tool and workpiece surface, emulsion clarity to allow operators a clear view of workpiece and lack of surface darkness or staining when the workpiece is aluminium. Compositions which can maximise one or preferably several, of these properties are desired. Additionally, the storage and viscosity properties of the metal working fluid concentrate are important for ease of handling and formulation.